of the Tr (metric System. 61 



II. Angle of brachydome near 109° 28'. 



III. Angle of macrodome near 109° 28'. 



IV. Angle of brachydome near 120°. 



The vertical axis in section II. is about one fourth shorter 

 than in section L; in the latter B = 85° 40', which approaches 

 li in the former, being very nearly the angle of Stephanite. 



Chrysoberyl is very near Aragonite in angle, if the plane in 

 the former usually regarded as fz be taken as K, as adopted in 

 the table : otherwise the relation for the vertical axes of the 

 two species is that of 3 : 2. So also Copper Glance approaches 

 Aragonite, if what has been taken by authors as fz be regarded 

 as 1% ; otherwise the relation between them is that of 2 : 3. 

 Such ratios, as we have elsewhere remarked, and the tables 

 everywhere illustrate, are consistent apparently with homoeo- 

 morphism in species. We have not sufficient data, at present, 

 to decide whether the relation between Aragonite and Copper 

 Glance is actually that of 1 : 1 or of 2 : 3, yet are inclined to 

 believe the latter the fact ; and if so, H in Copper Glance has 

 61° 54' for the summit angle, and 118° 6' for the basal ; the 

 latter angle is near that of the vertical prism. 



Many of the species in Tables I. to III. afford a horizontal 

 prism or unit dome of 115° to 120° ; and consequently, if this 

 dome were taken as the fundamental vertical prism, the species 

 would pertain to Table IY. Although we have not good rea- 

 son for making the change, it is of some importance to view the 

 species in this way, in order to apprehend more fully all the 

 affiliations and relations of the forms. The author has alluded 

 to Hausmann's comparisons by this method, of the anhydrous 

 sulphates and carbonates ; and he would here observe that the 

 general review of Trimetric forms which he has made since 

 his former paper was printed, and which has been here pre- 

 sented, has led him to give more importance to such com- 

 parisons than was implied in his paper in the American 

 Journal of Science, vol. xvii., p. 210. 



